Fix handling of Jibri busy response

[bbaldino]

This PR aims to address the following issue:

• Call A starts a recording, Jibri 1 is selected
• Call B starts a recording, Jibri 1 was selected (Jicofo didn't see it as busy yet)
• Call A's JibriSession gets a pending response
• Call B's JibriSession gets a busy response
• Jicofo treats any response that's not pending to a Jibri request as an error, so we now consider Jibri 1 in an error state
• Jicofo removes and re-adds Jibri 1 to the MUC in order to move it to the back of the line
• Call A's JibriSession sees the Jibri leave the MUC, so sends an error to the client (thinking Jibri went away)

The fix was to not treat the busy response as a transient error.